It was mentioned by others to, but really the point here is that "it is okay to make fun of those in power". There are power imbalances out there, that change how we react to a joke. Me making a joke about some person or group of people (or a characteristic robustly associated with said person/people) who are subjugated, is problematic. Me making fun of a person, people, or societal structure that has power over me, is the least I can do to resist. That's how we tease apart bad-taste jokes, from actually problematic jokes. Do they punch down, or do they punch up?

Of course that's assuming that the comment was indeed a joke, but trying to tell jokes and sincere opinions apart online is increasingly a frustrating task. To me it looked like very transparent trolling, but you can hardly now any more.

In short though, of course, if it's not a joke and someone sincerely holds the opinion that white genocide is a valid political program, of course that's problematic bullshit and clearly authoritarian. But it will get us nowhere to try and guess if they were joking all along, because of the medium. But find me a political group that claims to be anti-authoritarian and be pro-genocide, and of course things there will be pretty clear-cut.